In order to prevent accidents in and around machines and processes safety devices are e.g. needed to detect people coming into an hazardous space, coming close to a machine in operation, being in an hazardous area when a machine is put into operation etc. It is also necessary that hazardous machines and processes can be stopped in a safe way before a person enters the hazardous space, or is in a hazardous space close to a machine, before, but also at occurrence, of an incident etc.
A simple safety system can be based on a fence around a machine and an interlocked gate. If the interlocked gate is opened all hazardous functions must be stopped. An interlocked gate has a safety sensor which detects if the gate is opened and it will give a signal to a safety unit which stops the hazardous machines and processes. If someone actually can enter the space inside the gate and close, it is a requirement that it is possible to verify that no one is inside the fence before a new start of the machine can be allowed. This can be done by means of a push button outside the gate placed such that there is a good overview of the hazardous space. This push button is called a reset. After the gate is closed, a reset has to be done which normally means to press and release the push button. The pressing and releasing is supervised by a safety unit connected to the push button. Sometime it is hard to see from the outside of the fence that no one is inside the fence. This can be solved by an extra supervised time reset push button inside the fence in order to make sure that somebody goes in to check that no one is inside. This inside reset starts a clock with a pre-settable time period before lapse of which the space has to be left, the gate closed and the other reset be pressed.
This is one way to make sure there is no one left inside the fence.
Normally there is also provided an emergency stop push button outside the fence. In such a case the complete safety function to stop and to allow restart of the hazardous machines and processes is based on the control and the supervision of the following safety devices: a sensor on the gate, a reset push button outside the fence, a time reset push button inside the fence and an emergency stop push button.
The control and the supervision are normally based on one or more safety units or modules provided inside a control cabinet. The safety units or modules can have a fixed function as well as a programmable function. On the market safety modules are e.g. named safety relays, safety PLCs (safety Programmable Logic Controller), safety control units etc.
To install such a safety system, cables have to be connected from the control cabinet to the sensor, to the reset push button box, to the time reset push button box and to the emergency stop push button box and to the machines and processes.
Normally a safety system e.g. used within production additionally needs further safety functions such as additional supervised gates, supervised openings for detecting material for allowed transport into and out of the hazardous space, safety devices for setting up the production process and troubleshooting.
The control cabinet with the safety units or modules needs drawings, wires, marking of wires, DIN (Deutsches Institut für Normung)-rails for safety modules, terminals for cables, cable glands, and requires manual mounting and inspection in order to ensure safety that is provided and upheld according to regulations and standards. The control cabinet is costly and complicated to make and to make installations and connections is time consuming and complicated, and for a production line it is often unique and often involves machines from different manufacturers.
EP 1 496 411 shows a safety controller adapted to be mounted on a DIN rail in a control cabinet as discussed above.
When it is possible, companies are standardising on some safety functions in order to reduce the costs for the documentation and the inspection. This means, however, that changing from a standard solution is costly and complicated.
The requirements for the safety functions are described in safety standards globally and locally. In Europe the requirements are written in the Machinery Directive and in the EN standards for safety. The global standards for safety are in many cases in line with the EN standards. Large numbers of safety devices and safety modules have to be certified according to the safety standards before they can be used in Europe and also in other countries around the world.
If there is to be a combination of safety devices inside a control cabinet, a verification thereof is required as well. The costs for all the paperwork and inspection can be very high.
Thus, to summarize, there are several drawbacks associated with today used safety systems and safety units, such as the requirement as to specialized or customized control cabinets, high cost and time consuming documentation, planning and paperwork, certification and inspections. Other significant drawbacks consist in the large number of cables that are needed, and, in addition, the large number of wires inside the cables, which is very disadvantageous for installation and replacement purposes, the connection of each wire also needing to be verified, for practical reasons, and not least for reasons of trouble shooting. Still further, installation is complicated and time consuming, maintenance costs are high, and maintenance as such is complicated.
In addition, it is, among other things for the reasons given above, very complicated and time consuming to perform modifications, adaptations and alterations in/to such a known safety system, which means that, in practice, known systems have a limited flexibility.